Method of manufacturing light polarizing material



Patented Dec. 22, 1942 2,306,108 ICE METHOD OF MANUFACTURING LIGHT POLARIZING MATERIAL Edwin H. Land, Boston West Newton, Mass., assignors to Polaroid poration, Dover, Del., a. corporation of No Drawing. Original 8.

Serial No. 271,814.

9 Claims.

This invention relates to a method of manufacturing light-polarizing material.

An object of the invention is to provide a process for the production of a sheet or film of light-polarizing material consisting of a transparent plastic, having long molecules, and more specifically polyvinyl alcohol, with its molecules oriented to substantial parallelism, and which is difierentially absorbing to components of an incident beam of light vibrating in diflerent directions.

A further object of the invention is to provide a process for decreasing the heat sensitivity and the water permeability of light-polarizing material of the characterdescribed.

eral steps and the relation of one or more of such steps with respect to each of the others which are exempl fied in the follow ng detailed disclosure. and the scope of the application of which will be indicated in the claims.

efiicient light-polarizer which may be easily laminated to or between sheets of glass or sheets commercial applications of light-polarizing material.

Those of the commercial light polarizers now available which possess invention, a dry cast sheet or extruded sheet or film of polyvinyl alcohol may be heated and stretched at a temperature such that the molecules of and Howard G. Rogers,

Cor- Delapplication May 4, 1939, Divided and August 5, 1939, Serial No. 288,586

this application such as dyes, stains or diflerential absorption incident beam of light.

Many forms of polyvinyl alcohol, and including all examined, have been successfully employed in the practice of the present invention. For example, polyvinyl alcohol obtained from the R. & H. Chemicals Department of the E. I. du Pont de Nemours & Company, Inc., at Niagara Falls, New York, and designated A Series, and including one so-called low viscosity, three medium viscosity, and two high viscosity forms, and material obtained from'the same source and designated B Series, low, medium and high viscosity, has been employed, as has polyvinyl alcohol obtained from the Arlington d vision of the Du Pont Company. With all of these ma terials l ght-polarizing sheets are obtained by preparing sheets or films of poiyvinvl alcohol and heating t ese sheets to temperatures approximating 175 C. for from three to five m nutes, during which time the sheets are stretched or extended. The sheet may preferably be stretched several times its ori inal len th. for example from two to eight times its original len th. I

The mater al obtained from the A l n' ton division of Du Pont gives the best results. if t eated in the manner described. The stretch d and the like, the property of of the components of an heated sheet shows. upon cooling. a dark red color in the ext nction position when v ewed through an analyzer. and a substant ally neutral or colorless transmission when the anal zer is rotated through The materials obta ned from the R. & H. Chemicals Department s ow, on the other hand. yellow color in the ext nct on position when treated in the manner desc ibed.

It is desirable, especiallv in connect on w th the materials obtained from the R. & H. C emicals Department. to emp oy a mate ial w ich acts in the nature of a catal st to cause t e p oduct on of a darker and more des abl pola iz n extinct on color. Such a mater al w ll he called an accelerator. W th the A Series. for example. the sheet of polyvinyl alcohol ma he treated with a solution of 1% ac d. fo exam le hydrochloric acid or hydrobromic acid. or hyheated and stretched. the polarizer obtained is a deep blue. The efflciency of the polarizer is greatly increased, both as to extinction and also as to transmission, and the color appears at a somewhat lower temperature; for example at a temperature of about C. It is preferred to stretch the sheet at this temperature, although this is not important, as the color appears and the sheet may be stretched at the higher temperatures previously mentioned. I

With the materials obtained from the R. 8: H. Chemicals Department and designated "B Series," a similar acid accelerator may be employed. Here, however, the preferred solution is 1% acid in 60% methanol and 40% water.

With the use of the accelerators in connection with the R. 8: H. Chemicals Department polyvinyl alcohol, the polarizers obtained show substantially no color in the transmission position and excellent extinction with good color, high efli-- ciency and uniformity in the extinction position.

The accelerators may, if desired, be employed after the sheet has been stretched and the yellow color in the extinction position obtained. Under these circumstances the sheet should be reheated after the accelerator has been applied and held under tension during the reheating.

Under certain circumstances it may be desirable to heat and stretch the polyvinyl alcohol sheet in a hot atmosphere containing a high percentage of water vapor or superheated steam. The material obtained from the Arlington division, for example, gives an excellent polarizer showing a deep blue color in the extinction position, when heated to approximately 175 C. and stretched at that temperature in an atmosphere containing superheated steam. If desired, accelerators such asthose described may be employed with this form of material. If used they result in the development of the polarizing stain at lower temperatures. Unless care is taken the stain may appear too quickly and may not be uniform.

protected from moisture, as for example by the heat treatments described, and is highly emcient as a polarizer throughout the visible spectrum.

It shows no deterioration when subjected to long exposure to ultraviolet radiation. It has a neutral color in its preferred form to the transmitted component of incident light, and shows substantially complete absorption of the other component. It is substantially unaffected by most of the organic solvents for plastics and by the plasticizers normally employed in connection with laminating processes. When laminated it is substantially completely stable under the conditions It is frequently desirable to subject the polar- I izing sheets obtained as heretofore described to a further heating process. Such a process increases the impermeability to water of the product and increases its stability to changes in temperature. It has been found that further heat treatment, even at temperatures lower than those at which the sheet was formed, may result in a darkening of the sheet, not only in the extinction position but also in the transmission position, and thus to an impairment in its efficiency. This may be avoided if the sheet is first treated with an inhibitor. It has been found, for example, that if the sheet is soaked in a 5% solution of sodium chloride in equal parts of alcohol and water while maintaining it in stretched condit on. it may be subjected to the reheating treatment without discoloration. The solution ac s to nhibit change in the polarizing properties of the sheet.

So a so. if the sheet is laminated between two glass sheets. as for example with a bond compris n plastic zed polymerized incom lete polyvinyl acetal resin. further discoloration of the sh e is avoided. It will be apparent that other bond ng materials. such as vinyl acetate, methyl me hacrylate and the like, usually plasticized, may he u ed in ob a ning the lamination, or the sheet mav b l minated di ectly to glass without the use of other bonds or cements.

It sho ld be pointed out that the hi h tempe ure t which the sheet is subjected in conn ti n with the stret hing thereof acts effectively to substant a ly carry forward the p lymerz tion of the sheet. and to substant ally dec eas its permeability to water. In some commercial a pli at ons. further heat treatments, su h as those just described, may prove unnecessaw.

Th sh et nnlarizer produced by the practice of this inven ion may be easily laminated to or between glass, may be waterproofed or otherwise of use in commercial applications, and particularly in connection with the elimination of automobile headlight glare. While the invention has been described in connection with the use of polyvinyl alcohol, it should be understood that other plastic materials having elongated or chain-like molecules may be employed in the production of somewhat similar products. Cellulosic materials, such for example as regenerated cellulose, may be swelled by immersion in a suitable swelling solution until they have been rendered rubbery-elastic, as disclosed in the copending application of Edwin H. Land, Serial No. 237,783, filed October 29, 1938. Sheets of this material may then be stretched, for example, to the limit of their extension, and if they are then subjected to heat, for example at temperatures slightly below the charring temperature, a polarizing stain is developed in the material. It should be understood that such a product is not, however, as desirable as the products obtained by the use of polyvinyl alcohol.

In the practice of the present invention the stretching imparted to the heated sheet is preferably such as to substantially orient the molecules of the polyvinyl alcohol sheet. The final color of the polarizing sheet, especially in the extinction position, may, however, be controlled, in part at least, by controlling the degree of stretch. A sheet stretched, for example, to two and one-half times its original length, shows a noticeably different extinction color from a sheet stretched to five times its initial length. So also the temperature at which the sheet is stretched may be varied with a change in the extinction color of the sheet. All such modifications of the process are deemed to fall within the scope of the invention.

Furthermore, it is to be understood that a plasticizer may be added to the sheet to render it more flexible in its finished state. It is preferred that the plasticizer be added to the sheet in connection with its treatment by the inhibitor previously described, for it has been found that an unplasticized sheet may be more readily handled in the stretching process. Glycerine is a suitable plasticizer, as are ethylene glycol and formamid. They may be added in any desired amount, for example 10% by weight. Higher amounts tend to soften the sheet too much for most purposes.

The product of the present invention in its preferred form is substantially uniaxial with its optic axis in the plane of the sheet. It shows substantially no absorption for light vibrating perpendicular to its optic axis and high absorption for light vibrating parallel to its optic axis.

It is believed that the process of the present invention results in the dehydration of some of the long-chain molecules of the polyvinyl alcohol with the formation of hydrocarbon straightchain molecules having extended systems of con- Jugated iugated double bonds. Preferably the dehydration should be carried forward only until the sheet of polyvinyl alcohol has lost a small percentage, for example not more than 1%, of its initial weight. Under such circumstances, the sheet shows substantially no absorption for the component of the incident light vibrating transversely to the stretched direction of the sheet. If the dehydration is carried forward until the sheet loses substantially more in weight, for example 5%, more or less, the absorption of the sheet for the transversely vibrating component becomes appreciable within the visible spectrum, and may become undesirable, while the absorption of the component vibrating parallel to the stretched direction remains, as before, substantially complete.

The word stretching as used throughout the specification and claims shall be understood as including any elongation or extension of the material referred to whether accomplished by the application of tension thereto or otherwise.

Since certain changes in carrying out the above process may be made without departing from its scope, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

Having described our invention, what we claim as new and desire to secure by Letters Patent is:

1. A process of forming a dichroic light-polarizer from polyvinyl alcohol comprising substantially orienting the molecules of the said polyvinyl alcohol by stretching said polyvinyl alcohol and converting some of said molecules into oriented dichroic molecules having extended systems of conjugated double bonds by heating said polyvinyl alcohol at a temperature in excess of 150 C. and below the charring temperature of said polyvinyl alcohol.

2. A process of forming a dichroic light-polarizer from polyvinyl alcohol comprising substantially orienting the molecules of the said polyvinyl alcohol by heating and stretching said polyvinyl alcohol to at least twice its original length and converting some of said molecules into oriented dichroic molecules having extended systems of conjugated double size by heating said polyvinyl alcohol at a temperature between 150 C. and the charring temperature of polyvinyl alcohol.

3. A process of forming a dichroic light-polarizer from polyvinyl alcohol comprising substantially orienting the molecules of the said polyvinyl alcohol by stretching said polyvinyl alcohol at least twice its original length and converting some of said molecules into oriented dichroic molecules having extended systems of condouble bonds by heating said polyvinyl alcohol to a temperature below the charring temperature thereof and above 150 C. and treating said polarizer to decrease its water-permeability.

4. A process comprising forming a sheet of polyvinyl alcohol, orienting the molecules of said sheet by softening and stretching said sheet to at least twice its original length, and heating the said sheet to a temperature'below the charring temperature thereof and-at which a dichroic alteration product of polyvinyl alcohol is formed therein and maintaining the heating of said sheet at said temperature until said dichroic alteration product is formed therein,

5. In the process of producing light polarizers which polarize by differential absorption of the components of a transmitted beam, from transparent organic plastics having non-polarizing long chain molecules which are capable of being converted by dehydration into dichroic light-polarizing molecules, the steps comprising substantially orienting the molecules of the plastic by stretching the plastic and heating the plastic to a temperature below that at which it chars and until some of the molecules thereof are dehydrated and converted into dichroic light-polarizing molecules.

6. In the process of producing light polarizers which polarize by diiferential absorption of the components of a transmitted beam, from trans parent organic plastics having non-polarizing long chain molecules which are capable of being converted by heat into dichroic light-polarizing molecules, the steps comprising substantially orienting the molecules of the plastic by stretching the plastic and heating the plastic to a temperature below that at which it chars and until some of the molecules thereof are converted into dichroic light-polarizing molecules.

'7. In the process of producing light polarizers which polarize by differential absorption of the components of a transmitted beam, from transparent organic plastics having non-polarizing long chain molecules which are capable of being converted by heat into dichroic light-polarizing molecules. the steps comprising substantially orienting the molecules of the plastic by stretching the plastic and heating the plastic at a temperature below that at which it chars and until some of the molecules thereof are converted into dichroic light-polarizing molecules.

8. In the process of producing light polarizers which polarize by differential absorption of the components of a transmitted beam, from transparent organic plastics having non-polarizing long chain molecules which are capable of be ng converted by heat into dichroic light-polarizing molecules, the steps comprising substantially orienting the molecules of the plastic by stretching said plastic and heating said plastic in the presence of an accelerator to a temperature below the charring temperature of said plastic until some of the molecules thereof are converted into dichroic light-polarizing molecules.

9. In the process of producing light polarizers which polarize by differential absorption of the components of a transmitted beam, from transparent organic plastics having non-polarizing long chain molecules which are capable of being converted by heat into dichroic light-polarizing molecules,- the steps comprising substantially orienting the molecules of the plastic by stretching said plastic and heating said plastic in an atmosphere containing water vapor to a temperature below the charring temperature of said plastic but sufliciently high to convert some of said molecules into dichroic light-polarizing molecules.

EDWIN H. LAND. HOWARD G. ROGERS- 

